Lead researcher Dr Mark Ware, director of clinical research at the Alan Edwards Pain Management Unit, McGill University Health Centre, said: "We found that 25mg herbal cannabis with 9.4% THC, administered as a single smoked inhalation three times daily for five days, significantly reduces average pain intensity compared with a 0% THC cannabis placebo in adult subjects with chronic post-traumatic/post-surgical neuropathic pain."

No serious adverse effects occurred during the trial. All the patients taking active cannabis reported one occasion of feeling "high" and euphoric, and some complained of headaches, dry eyes, a burning sensation, dizziness, numbness and coughs. No significant changes in vital signs, heart rate or kidney function were recorded.

The findings, reported in the Canadian Medical Association Journal, CMAJ, are believed to be the first from a clinical trial of smoked cannabis. The authors recommended more studies with higher potencies of THC, longer follow-up periods, and flexible dosing.